High frequency switching power supply designs have inherent limitations that limit the voltage repeatability in a non-quiescent state, such as capacitor charging applications. These limitations include EMI (electromagnetic interference), charge quantization, output current ripple, and program and feedback voltage integrity. Precise voltage regulation of capacitors, on the order of less than 0.05 percent, in a pulse-to-pulse charging mode of operation, which store less than 0.002 percent of the average energy of the power supply, has been difficult to achieve. The ability to stop output current on command is limited to inefficient, non-resonant pulse width modulation switching topologies. If such an approach is used, the high power capability of the power supply inherently creates a tendency to overshoot the objective due to the energy established in real and parasitic inductances within the power supply and in the connecting circuits to the load capacitor. Minimizing the inductance and remote voltage sensing at the load capacitor are not sufficient to reduce the effect to meet the desired precision in voltage regulation and repeatability.
When a resonant topology is used, the ability to stop output production is limited to ending on a resonant cycle, resulting in a quantization of output current into packets that establish the fundamental limit of the minimum size increment of load capacitor voltage. For frequencies less than 200 kHz, the limitations on switching frequency produce a packet size that is too large. The decision to stop charging is accomplished by comparing the programmed voltage to the feedback voltage, which are scaled to the order of ten volts for full output voltage. This scaling results in a 5 mV decision for a voltage resolution of less than 0.05 percent. The dV/dt electrical noise generated by the power converter and the magnetic coupling from the circulating currents (at any output current level) is significantly greater than this 5 mV level.
Although it is possible to remotely send an analog signal with less than 1 mV of noise by filtering it, the requirements of a bandwidth greater than 5 kHz do not allow it. Electrical noise injected during the charging period is integrated by the filter and produces a varying offset voltage.
The voltage regulator in the present invention addresses this problem by delaying the precision voltage comparison decision until after the power conversion has stopped. By intentionally allowing the output to overshoot a prescribed amount, after which time a precision shunt regulator is engaged, the output capacitor voltage is lowered in a substantially linear manner.